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Industrial automationIntermediatePLCprogrammable controllerGRAFCETOmronsequentialHMI

PLC and GRAFCET

In 30 secondsA PLC cyclically reads all its inputs, runs the program and updates the outputs. GRAFCET describes the process as a sequence of steps and transitions: each step waits for a condition before moving to the next.

Also available in: Català · Castellano

The PLC

A programmable logic controller is an industrial computer designed to control machines. It works in a cycle that repeats very quickly:

  1. Read all the inputs (digital and analogue sensors, push buttons).
  2. Run the program.
  3. Write the outputs (solenoid valves, motors, lights).

GRAFCET

A graphical way of describing a sequential automation:

  • Steps: what the machine is doing at that moment (for example, “piston A out”).
  • Transitions: the condition for moving to the next one (for example, “S1 = 1”).

For Phase 1 of the cell: feed → is there a part? → transfer → sort → measure height → pass to the conveyor or reject. Each phase is structured as a subroutine.

Two PLCs in the cell

  • General PLC: coordinates the three phases.
  • Servo PLC: controls the X-Y manipulator’s drives and the safety light curtains.

They communicate through link bits. Splitting the work relieves the main PLC of the positioning tasks.

HMI

The touch screen is used to start the cell, see the measurements, place orders and manage user access.

Interactive demo

Related concepts

Industrial automation

Flexible manufacturing cell

An automated line in three phases: feeding and sorting, machining and storage. Two PLCs coordinate sensors, pneumatic cylinders and servomotors to handle each part according to its material and colour.

Actuators

Pneumatic cylinders and solenoid valves

Compressed air moves the cylinders; solenoid valves are the bridge to the PLC. A monostable valve springs back to rest on its own when it loses the signal; a bistable one needs a signal for each movement and remembers its last position.

Control

Closed-loop control (PID)

A PID compares what you want with what you measure and corrects using three terms: P looks at the present, I at the past and D at the future.